The invention relates to a pressure-medium actuated vehicle braking system have separate braking circuits for the front and rear axles.
A braking system of this type is known from EP-A 0 079 543.
This known braking system comprises a dual circuit braking system wherein a relay valve system is assigned to the brake cylinders of the front axle as well as to the brake cylinders of the rear axle for the control of the braking pressure.
The division of the braking system into two braking circuits ensures, in case of the failure of one braking circuit, that the braking cylinders of the other braking circuit are still supplied with pressure medium, so that the vehicle can be decelerated with relative safety.
In case of failure of the braking circuit serving as the rear axle braking circuit, good deceleration of the vehicle is still achieved, since due to the dynamic axle load displacement during a braking event, approximately 70 to 80% of the braking force is produced by the wheels of the front axle of the vehicle.
However, if the braking circuit assigned to the wheels of the front axle of the vehicle fails, it may be difficult to reach the braking force that must be produced by the wheels of the rear axle of the vehicle as prescribed by law. This may occur particularly when the vehicle has a very low empty weight and a high admissible overall weight, since the wheels of the rear axle of the unloaded vehicle are not subjected, under unfavorable circumstances, to a force in the direction of the road surface which would ensure good deceleration of the vehicle.
It is therefore the object of the invention to provide a vehicle braking system actuated by a pressure medium of the type mentioned initially, and which is designed so that reliable deceleration of the vehicle is ensured even under unfavorable road and weight conditions for the vehicle and even in case of a failure in one braking circuit of the vehicle.